The present invention relates to an apparatus and a method for measuring time intervals. Usually time intervals are measured by counting clock pulses spaced apart in time by a known period for determining the time interval between a start event a stop event. As is commonly known, this approach allows for determination of the time interval only to a precision of plus or minus one count of the period of the clock pulses. To increase precision, it is desirable to measure the time elapsing between the start event and the occurrence of the first counted clock pulse (start time) as well as the time elapsing between the stop event and the occurrence of the last counted clock pulse (stop time).
To this end, known instruments use techniques to stretch the start and stop time intervals. According to a known method as disclosed in Hewlett-Packard Journal, Vol. 20, No. 9, May 1969, pp. 9-12, a capacitor is charged by a constant current during the time between the start or stop event and the first following clock pulse. Upon occurrence of the clock pulse, the time needed to discharge the capacitor with a lower current is measured by counting clock pulses. This time is proportional to the charging time interval by the factor of difference in currents and may be combined therewith to provide the time between the event and the first following clock pulse.
This method has the disadvantage that in measuring short time intervals, the time spent in stretching the start and stop time intervals may be considerably longer than the time interval between the first and last counted clock pulses. This disadvantage leads to a limitation of the rate at which the measurement can be repeated.